In a vehicle having four wheels, rear wheels or front wheels are used as driving wheels, and power of a prime mover such as an engine is transmitted thereto. Meanwhile, in case the vehicle is a four-wheel drive vehicle, power outputted from the prime mover is transmitted to all of the four wheels. In order to ensure running stability of the vehicle, the driving forces for driving the driving wheels are equalized in case the vehicle is running straight ahead. However, in order to maintain the stability of the vehicle in case frictional coefficients of a road surface against the left and drive wheels are different from each other, or in order to enhance turning characteristics of the vehicle utilizing a yaw moment, the driving forces for driving the driving wheels are controlled to increase the yaw moment.
For example, Japanese Patent Laid-Open No. 2007-127145 discloses an apparatus for controlling driving forces for driving right and left wheels. Specifically, according to the teachings of Japanese Patent Laid-Open No. 2007-127145, power generated by a driving power source such as an engine is distributed to the right and left driving wheels, and a speed reducing mechanism and a clutch are arranged individually between the right wheel and a motor and between the motor and the left wheel. Therefore, the power outputted from the engine is distributed and transmitted to those wheels. Meanwhile, power of the motor is transmitted to any one of the right and left driving wheels by engaging any of the clutches. Consequently, a difference between the driving forces for driving the right and left driving wheels is widened. In other words, a substantial distribution rate can be altered depending on a situation.
Meanwhile, Japanese Patent Laid-Open No. 2000-94979 discloses a vehicle in which any one of the pairs of front and rear wheels are driven by an engine, and the other pair of wheels are driven by a motor. Because a response of the motor is more rapid than that of the engine in case of controlling drive force and braking force, according to the teachings of Japanese Patent Laid-Open No. 2000-94979, the drive force and the braking force are compensated by the motor.
According to the apparatus taught by Japanese Patent Laid-Open No. 2007-127145, the difference between the drive forces for driving the right and the left wheels are controlled by engaging or disengaging the clutch to apply a motor torque to one of the right and the left wheels, or to stop applying the motor torque. The clutch is conventionally engaged and disengaged hydraulically, however, such a hydraulic clutch control may be delayed inevitably. Therefore, in order to control a behavior of the vehicle such as a yawing accurately, such control of the clutch has been required to be improved.
For example, a quick-fill for applying oil pressure rapidly in the beginning of the hydraulic control, a phase-lead control and so on are known in the art as a solution to avoid a delay in the hydraulic clutch control. However, although transient response of those controls is excellent, a control amount of those controls may be fluctuated in case a control input is varied drastically. Therefore, stability of those controls has to be improved.